CD4+ T helper (Th) cells can be divided into two functionally distinct subsets: Th1 cells, which produce interferon-gamma upon activation and promote cell-mediated immunity, and Th2 cells, which are important for humoral immunity because of their ability to produce cytokines such as interleukin-4 (IL-4) and lL-5. The Th1/Th2 balance of any given immune response is perhaps the most important predictor of the outcome of that response, and an imbalance between these two Th cell subsets is strongly correlated with diseases ranging from autoimmunity to asthma. Thus, understanding the molecular mechanisms that regulate the differentiation of Th cell subsets is of critical importance. Although a number of factors are thought to dictate whether a naive Th cell differentiates into a mature Th1 or Th2 cell, it is the cytokines themselves that are arguably the most important in influencing this lineage decision. In particular, IL-12 has been shown to be required for the generation of Th1 cells, while IL-4 is critical for the generation of Th2 cells. Over the last several years, significant progress has been made at understanding the molecular mechanisms by which lL-4 regulates Th2 cell development. Indeed, studies from our laboratory and others have demonstrated that IL-4-mediated activation of the signal transducer and activator of transcription 6 (Stat6) signaling pathway is a critical step in the generation of Th2 cells. Despite this progress, important questions regarding the mechanisms that govern the generation of Th2 cells, and the factors that control their effector functions, remain unanswered. The overall goal of this grant application is to further understand how the differentiation and function of Th2 cells are regulated at the molecular level. The experiments proposed examine several aspects of Th2 cell biology, including the role of Stat6 in polarized Th2 cell responses, the biologic consequences of constitutive Stat6 expression, and the function of a novel Th2 cell-derived cytokine, IL-21.